Not applicable.
This invention relates generally to radar transmitters and more particularly to a radar transmitter having a digital ramp signal generator and temperature compensation, non-linear VCO compensation, and reduced interference features.
As is known in the art, there are many types of radar transmission techniques, one of which is Frequency Modulated Continuous Wave (FMCW) transmission, in which the frequency of the transmitted signal increases linearly from a first predetermined frequency to a second predetermined frequency. FMCW radar has the advantages of high sensitivity, relatively low transmitter power and good range resolution.
Various circuitry and techniques can be used to generate an FMCW transmit signal. One technique is to feed a signal voltage having a ramp characteristic (referred to herein as a
Various circuitry and techniques can be used to generate an FMCW transmit signal. One technique is to feed a signal voltage having a ramp characteristic (referred to herein as a xe2x80x9cramp signalxe2x80x9d or ramp voltagexe2x80x9d) to a voltage controlled oscillator (VCO) to generate the frequency modulated transmit signal commonly referred to as a chirp signal. Typically, the ramp signal is generated by an analog circuit which may include timing pulse generation circuits, integrators and amplifiers. Components of such an analog circuit are fixed at the design stage and thus, such a circuit does not afford much, if any versatility.
Ideally, the frequency of the VCO output signal varies linearly with respect to the ramp voltage. When there is non-linearity in the ramp signal and/or in the operation of the VCO, the frequency of the RF return signal can be spread across an RF frequency range or xe2x80x9csmearedxe2x80x9d, thereby degrading target detection, resolution and range accuracy performance of the radar system.
Another technique for generating an FMCW transmit signal is to use direct-digital synthesis (DDS) in which the transmit signal itself is digitally synthesized. Typical DDS systems include a phase accumulator and a digital-to-analog (D/A) converter. However, the transmit signal rate is limited by the Nyquist theory to less than one-half of the maximum clock rate of the D/A converter. Other disadvantages of DDS systems include complexity and cost plus an increase in supporting hardware requirements because of limitations in operating frequency and tuning range of currently available DDS synthesizers.
As is also known, some relatively complex radar systems include multiple transmit and receive circuits (TRCs) each of which operate independently of one another. When such transmit and receive circuits are placed in proximity to one another and operate at the same or overlapping frequencies, the multiple TRCs can interfere with one another, preventing the accurate detection of targets. Other problems can also arise by simultaneous operation of multiple TRCs.
Radar systems provide several design challenges. As one example, when radar systems operating at the same or overlapping frequencies are used in proximity to one another, the two systems can interfere with one another, preventing the accurate detection of targets. For example, circuit performance variations attributable to temperature changes can result in interference between multiple TRCs. It would, therefore, be desirable to provide a radar transmitter circuit which permits adjustment of transmit signal characteristics in a relatively simple manner. It would also be desirable to provide a radar transmitter which compensates for variations in transit signal characteristics caused by variations in temperature in the environment in which the radar transmitter is disposed. It would be still further desirable to provide an FMCW radar system which compensates for non-linear VCO operation. It would be still further desirable to provide a technique which allows simultaneous operation of multiple TRCs in overlapping frequency ranges. It would be still further desirable to provide a system and technique which allows simultaneous operation of multiple FMCW TRCs.
In accordance with the present invention, a radar transmitter includes a DSP, a D/A converter having an input terminal coupled to the output terminal of the DSP and an output terminal at which an analog ramp signal is provided, and a VCO having an input terminal responsive to the analog ramp signal and an output terminal at which a frequency modulated signal is provided.
With this arrangement, simple and relatively inexpensive circuitry is used to generate an analog ramp signal for controlling the VCO. Several advantageous features can be readily implemented by appropriate adjustment of the DSP output words, which results in concomitant adjustment of the chirp signal. These features include VCO and drive circuit temperature compensation, compensation for non-linear VCO operation, and interference reduction techniques. An analog smoothing circuit may be coupled between the output terminal of the D/A converter and the input terminal of the VCO in order to smooth the stepped D/A converter output.
In one embodiment, the output of the VCO is up-converted to provide the transmit signal and in another embodiment, the VCO operates over the transmit signal frequency range, thereby eliminating the need for the up-converter.
Also described is a VCO which consists of a dielectric resonator oscillator (DRO) to generate the chirp signal. The VCO includes an amplifier, a dielectric resonator (DR) for controlling the center frequency of the VCO and a phase shifter for providing a frequency tuning capability to the VCO. The phase shifter is a three terminal device which has an input terminal coupled to the amplifier, and an output terminal connected to the dielectric resonator. The dielectric resonator is connected back to the input of the amplifier to provide positive feedback and thus create an oscillator. In addition, the phase shifter has a control terminal to control the frequency of the VCO by providing a phase shift proportional to the control voltage. The frequency modulating signal or xe2x80x9cramp signalxe2x80x9d is connected to the control terminal which is responsive to the ramp signal.
A temperature compensation feature is described including the steps of generating, from a predetermined sequence of digital words, a transmit signal having a frequency associated with a respective one of the sequence of words, and storing each of the digital words in association with an expected transmit signal frequency. The actual frequency of the transmit signal is detected and the digital word used to generate the detected frequency is compared to the digital word stored in association with the actual transmit signal frequency. The result of the comparison is an error value which is used to adjust each of the digital words. In one embodiment, the actual transmit signal frequency is detected with a circuit which is responsive to a narrow band of frequencies and the digital words are adjusted by introducing an offset equal to the error value. In one embodiment the circuit is provided as a DRO which is responsive to signals having the transmit signal frequency.
According to a method for compensating for non-linear VCO operation, the VCO is characterized during manufacture by feeding a predetermined sequence of digital words to the D/A converter and detecting the resulting transmit signal frequency for each word. This process yields a so-called VCO curve which relates VCO output frequency to VCO input voltage. A curve having a shape which is complementary to the shape of the VCO curve is determined and the DSP output words are adjusted to provide the complementary waveform to the VCO. By controlling the VCO with a waveform complementary with respect to its characteristic curve, frequency smear caused by such non-linear operation is reduced.
A method for reducing interference between radar systems includes the steps of generating a ramp signal for controlling a VCO and randomly varying at least one parameter of the ramp signal. The ramp signal includes a plurality of cycles, each having an offset portion, a ramp portion, and a CW portion. The parameter of the ramp signal may be randomly varied in one or more of the cycles. Illustrative ramp signal parameters which may be randomly varied include, the starting ramp signal voltage, the duration of the offset portion and the voltage range of the ramp portion.